Black printer



Dec. 19, 1939. J c, YULE' 2,183,525

BLACK PRINTER Filed Feb. 18, 19:9 '2 Sheets- Sheet 1 Fig.1.,

ORIGINAL I I0 Q Q 0 EXPOSING AND GREEN 6 PROCESSING. STEPS COLOR I ISEPARATION I I2 I NEGATIVES PRINTING AND PROCESSING TO HIGH CONTRAST I47LIB I [:6

PRINTING SUCCESSIVELY I TO ADD EXPOSURES HIGH CONTRAST POSITIVES I PHOTOI I SENSITIVE LI7Aj- L 175 H 17c- LAYER PROCESSING T0 LOW CONTRASTNEGATIVE 179 PRINTING AND PROCESSING POSITIVE OR PRINTER 8 (BLACKPRINTER) Dec. 19, 1939. J. A. c. YULE BLACK PRINTER Filed Feb. 18, 19392 Sheets-Sheet 2 V 5 Fig.2.. BLUE MODIFICR' 23? 24 CIRCUIT LIGHT HQEAIIPIII'IER 25 k AMPLIFIER VALVE 20 21 I (roRYELLow NTH --2 PRINTER)POWER I AMPLIFIER GREEN Q cIRcuIT I MODIIIIER I c;

- I LIGHT J] IZAIIPIITIE 26 R AMPLIFIER fi VALVE a L roR MAGENTA 1 27GPRINTER) POWER AMPLIflfR 22 R 23R I RED CIRCUIT LIGIIT H Pllflf umfivALvt R zI I (FOR 20 BLUE-GREEN PRINTER) LIGHT IIIIPIITITR VALVE 29-AMPLIFIER 31 30 FOR BLACK PRINTER grvuezwto'b J OhTl A.C.YU1

Patented Dec. 19, 1939 PATENT; OFFICE 4 2,183,525 BLACK PRINTER John A.C. Yule, Rochester, N. Y., assignor to Eastman Kodak Company,corporation of New Jersey Rochester, N. Y., a

Application February -18, 1939, Serial No. 257,227

17 Claims.

This invention relates to photomechanical color reproduction. It relatesparticularly to black printers for use in four color 'processes and tomethods of making such printers.

It is an object of the invention to provide a highly corrected blackprinter negative and hence a highly corrected .black printer.

It is also an object of the invention to provide a method of making ablack printer which may be performed either photographically orelectrooptically,

To avoid ambiguity regarding the various nomenclature conventions, theone here used will now be outlined. The primary colors are red, green,and blue. The refiectivities, to the three primary colors, of any pointof the original which is to be reproduced are: Rr, Ry, and Rb. Thecorresponding printing inks are the subtractive primaries. blue-green,magenta and yellow.

The color separation negatives made through the three primary colorfilters have at each point, densities: Dr, D9, and Db. If a blackprinter negative is made, its corresponding density is defined as Dr. Ifcorrection is introduced corresponding to the masking method of colorcorrection, these densities are called Ar, Ag, Ab, and Azv. Positivesmade from each of these negatives will have the respective densities,D'r, D'g, Db, and D's. These densities will be (for purposes ofcomputation) equal to the printing densities of the final printers.These corresponding printing densities are defined as Dbg, Dm, D11, andDblack. Following the usual rule that density though the printers arenot actually transparent, it is convenient for the purposes of thepresent invention to speak of the printer transparencies,

which are related to the printing densities according to this rule. Whenthe original is reproduced by a. three-color process thesetransparencies are defined as Tbg, Tm, and Ty. These transparencies arepreferably but not necessarily those of printers produced by processesinvolving color correction corresponding to masking. In four-colorprocesses the transparencies. are defined as. T'bg, Tm, Ty and Tw.Obviously at any point where the black printer prints, T'y is greaterthan Ty, because part of the printing density has been assigned over tothe black printer which has a transparency Tx.

' In actual practice theblack printer is often used with colorprinterswhich have not been reduced, 1. e. with printers which are correct for athree color process.

In addition to defining the nomenclature used, it is necessary to givean exact definition to some of the terms used in this specification andthe .18, accompanying claims. This is particularly necessary since theinvention may be practiced equally well with photographic orelectro-optical systems. In the simplest form of electro-opticalsystems, for example those used in color telel0 vision, the original tobe reproduced is scanned in the three primary colors and signalscorresponding to the respective colors are set up in the red, green andblue channels ofthe electrical circuit used. These signals correspond tothe. 15 images of the color separation negatives made in a photographiccolor process. In both systems these will be spoken of as colorseparation records of the original. The electro-optical signals arepositive records whereas the color sepa- 20 ration negatives arenegative records.

In a copending application Serial Number 120,964, filed January 16,1937, Murray and Morse describe a method of introducing color correctionin such an electro-optical system. In a co- 2 pending application SerialNumber 234,422, filed October 11, 1938, Hall describes the use ofnonlinear amplifiers in electro-optical color systems. For convenience,I define the effect of a nonlinear amplifier, which amplifies to thepower N, as changing the contrast of the records. That is, thenon-linear amplifier introduces a contrast N. This is particularlyuseful terminology since the change in contrast of therecords-corresponds exactly to the effect of developing the colorseparation negatives in a photographic process to a contrast N.Incidentally, electro-optical systems having linear amplifiers withvarious mutual conductance values do not aiiect the contrast since thesignals correspond to exposures, 40 (i. e. correspond to refiectivities)and hence the factor introduced by a different mutual conductancevaluecorresponds merely to adding or subtracting a constant density. A changein contrast on the other hand involves multiplying 45 the densities by afactor.

The "adding ofv separation records linearly, which is an importantfeature of the present invention, also requires a special definition tocover both photographic and electro-optical sys- 60 tems and will befully understood from the detailed description of the two embodiments.

' A brief history of the development of corrected black printers isnecessary for the understanding of my invention. Parallel with thedevelopment I 01' color correction wherein the amount of one color whichis printed is modified in accordance with the amount of another color,has been the development of black printers. Because of the 5 hope thatcolor reproduction processes would some day be developed to the stagewhere black printers are unnecessary and because the amount ofcorrection gained by the use of a black printer is relatively less thanthat obtained by correction of the color printers, the development ofthe black printer has, until recent years, apparently lagged behindother types of color correction.

Theoretically there are two ideal types of black printers andpractically, any form of black printer between these two typesis-useful. What constitutes an ideal black printer depends, of course,on other features of the process in which it is used. If the three colorprinters are arranged as in a three color process to reproduce theoriginal, 20 including the blacks, as nearly as possible, an ideal blackprinter is one which merely takes care of the deficiencies in thethree-color process. The other extreme is the preferable one wherein allblacks and grays are reproduced by the black 25 printer and the colorprinters have printing densities reduced from that which they had in athree-color process, by the amount which is taken care of by the blackprinter.

One of the earliest and even now one of the so commonest methods ofmaking a black printer negative is to expose a panchromatic film orplate to the original through a light yellow filter. It is well knownthat this does not produce a good black printer, i. e. one in which allsaturated col- 35 ors have been eliminated without loss of the lighttones in the gray scale. Mathematically this exposure E=ClRg+C2Rr o inwhich C1 and C: are constants. Therefore,

where 1 is the contrast to which the negative is developed.

An equivalent system was proposed by Albert 45 in 1897 in his GermanPatent 101,379 wherein he successively exposes through no filter,through a red filter and through a green filter. This adds a verysmallamount of blue to his original exposure and hence difiers slightly fromthe mathematic equation given above. Actually however a yellow filteralso passes a very small amount of blue and hence the two systems areequivalent and the equation should contain a blue term. The blue effectis however relatively small.

The following year Albert (German Patent 116,538) set up what hebelieved to be an ideal black negative and attempted to produce it bycombining a ,corrected positive with an uncorrected negative. Thepositive corresponded to a yellow filter and the negative to a bluefilter. All of the variations which he described are mathematicallyequivalent to his original mask, i. e. the one used in making hiscorrected positive, or equivalent to this mask combined with a fiatuncorrected negative or positive. Subsequent theories and practice haveshown that such methods are applicable only to the reproduction of originals containing a limited range of colors and even then there is onlylittle improvement over the yellow separation negative. method.

Selecting one type of black printer as representative oi the innumerableimprovements discovered empirically during the subsequent years,attention is directed to U. S. 1,576,118, wherein 1| Hahn makes a blacknegative as follows:

A red filter negative mask is combined with the original (or rather theoptical image of it), a negative is made from the combination using agreen filter, a supplementary exposure being given with and filter andno mask. The equation for the exposure received by the negative is:

where gamma 7 equals the contrast of the mask, assuming that it is onthe straight line portion of the characteristic curve, C: and C4 areconstants. If gamma has a suitable value, RQ/R f is the transmission ofa positive corrected as in masking with an overall contrast (compared tothe original) of 1'y. This method appears to be an advance over theyellow filter negative method since a corrected instead of theuncorrected green separation record is used. Otherwise there is littlediiference between the two.

In common with the simple yellow filter negative method, Hahn's systemis applicable only over a limited range of colors. For example if onepoint 'of the original has a violet color, the black negative will atthat point receive only about of the correct amount of light.

In a copending application Serial Number 44,125, filed October 8, 1935,Murray suggests the use 0! infrared instead of yellow in making a blackprinter negative. For most color pigments in the original, 1. e. all ofthose which are transparent in the infrared, this gives a black printerunaffected by the colors. If the original is paint- 1 ed so that allgrays or gray components are made up entirely of a black pigment whichabsorbs infrared, (i. e. at no point are the three subtractive primariesmixed) such a black printer is of the second ideal type taking care ofall grays. When the original has grays made by mixing the threesubtractive primaries, these grays are reproduced by the color printersrather than the black printer.

In my 'copending application Serial Number 138,351, filed April 22,1937, I described a black printer whose printing. density at each pointis equal to the least predominant subtractive color content of the colorof the corresponding point of the original. Such a printer takes care ofall grays and gray components of each color in the original independentof their physical composition. In that application I describe two of themost promising methods of making such a printer. Hall and Morse describea third method in their application Serial Number 215,822, filedJune-25, 1938. No methods of producing such a printer were known priorto my invention. Both practically and theoretically such printers aresuperior to any previously produced and are equally useful with anyrange of colors (except for second order efiects) Murray's infrared"black printer is for most practical purposes equivalent to this one whenused with a suitable original, but of course difiers therefrom in theway discussed above.

- ency at each point is Ta: where where Tby, Tm. and Ty arethe printedtransparencies of a three-color process as described above and N is anumber equal to or greater than 1.0 and preferably greater than 1.5.Such a printer may be used either with the color printers intended for athree-color process or preferably with color printers in which thedensities have been reduced by the amount which. is to be printed bythis black printer,

From my copencling application method above, it is obvious that theblack printer printing transparency should be equal to the printingtransparency Tbg,' Tm, or Ty whichever is the greatest. Mathematically,it is obvious that the Nth root of the sum of the Nth powers of anythree numbers will be approximately equal to whichever is greatest ofthe three numbers as N tends to infinity. For example if one of thenumbers is large compared with either of the others, it is obvious thatthe sum of the Nth power of the three numbers is not very much greaterthan the.

Nth power of this large number alone. Even under the worst possibleconditions. i. e. that "when the three numbers are absolutely equal, the

Nth root of the sum of the Nth powers of the three numbers will only bethe Nth root of 8 times one of the numbers.

. Applying this methematical phenomenon to black printers it has beenfound that a black printer whose printing transparency is gives betterquantitative results than the yellow filter negative method even when Nequals 1 if color correction corresponding to masking has been includedin the establishing of the color printer transparencies and even whenused with color printers intended for a three-color process. Where nocorrection equivalent to masking is used, N should be slightly greatersay N=1.3 orI 1.4 to get results as good as the yellow filter negativemethod which is unrelated to this inven tion but which forms aconvenient standard oi! quality. WhenN equals 1,

factor, discussed above, which occurs for pure grays wherein T'bg=Tm'=TuIn the case of pure gray T'x should equal one of these whereas a systemwhich has, N=3 gives:

Although pure grays are somewhat rare and any color differing by anyappreciable degree from gray is satisfactorily reproduced, one maycompensate for the error in grays by some'form of compromise. Forexample TIX: N b0 m u where P has some value between 1' and 3. For

pure gray P should be 3 and for any pure color P should be 1. If N is atall large, even colors only slightly different from gray require a P notvery different from 1. With N between 2 and (e. g. N=3), I have foundthat P 1.2 gives a good compromise for an ordinary range of colors.Obviously this differs only slightly from the more general case where Pequals 1. 1 and 2, I have found that P=1.5 is best. In general P must bebetween 1 and 3.

Figures 1 and 2 show respectively a photo graphic and an electro-opticalembodiment of my invention.

In both cases, the black printer separation negative is produced byforming red, green and blue positive separation records of the originalwith a contrast N with respect to the original where N is a numbergreater than 1, by adding the three records linearly to give acombination record and .by forming from this combination record a photo-With N between graphic negative having a contrast substantially I l/Nwith respect to the combination record. A printer may be made from thisnegative, ,in any of the well known manners either by printing directlyor by making an intermediate positive. The first step of this processwhich involves the making of high contrast positive records of theoriginial may or may not include an intermediate positive or negativerecord. This first-step preferably includes some form of colorcorrection such as or equivalent to masking.

This gives a black printer whose printing transparency Tfa: is

and whose printing density D black is according to the equationComputing back from .this it is seen that the black printer negative hasa density where C5 is constant. This equation is easily visualized fromeither of the embodiments shown. The normal positive transparencies Tbg,Tm and Ty are separately raised to the' power N (i. e. given thecontrast N). Then they are added linearly and constitute the exposure ofthe black printer negative which is made to have a contrast with respectto this exposure. This is obviously the interpretation .of the lastequation and the description of the embodiments shown will be seen toconform exactly with this interpretation.

Incidentally since the final result is to have T'x dependent only on thelargest of the color printer transparencies, thesame efiect could begained by having a A: equal to the density of the largest of the threeprinter negatives, Ar, Ag and Ab, i. e.

Although the twoequations for black printer negatives give resultswhichare equivalent practically, they are obviously not the samemathematically.

In the embodiment shown in Figure 1 a multiand l 6. The high contrastrecords which constitute the images in the positivesv are then added byprinting successively onto a sensitive photographic layer shown in thisfiow chart at the positions "A, "B and "C while it receives therespective exposures. These exposures are spoken of as being equal, butthe actual time of each exposure depends, of course, on the actualdensity of the negatives and the positives. After receiving these threesuccessive equal exposures the photosensitive layer is developed to alow contrast to form a negative "D. The contrast of this negative "Bshould have a value with respect to the positives ll, l5, and I6 whichis substantially reciprocal to that of the value of the contrast of thepositives l4, l5, and IS with respect to the original I. The negativeI'ID forms the black printed negative and a'black printer or a blackprinter positive 18 may be made therefrom in the usual manner. In thepreferred use of my invention, this black printer is employed with colorprinters whose densities have been proportionally reduced.

- In the embodiment shown in Figure 2, the original, not shown, isscanned by any of the well known methods used in electrooptical systems.and the light is passed through blue, green and red filters 20B, 20G and20B respectively to phctocells ZIB, ZIG and HR to set up signals in theusual way in the blue, green, and red channels. These signals whichconstitute positive color separation records of the original in theusual manner of electro-optical systems are then suitably amplified byamplifiers 223, 22G, and 22R respectively.

If desired, color correction may be introduced in the manner taught byMurray and Morse in their copending application mentioned above, bysuitable modifiers and 26. Such correction is. however, not absolutelynecessary to or part of the present invention. The signals then passthrough amplifiers 233, 23G and 23R to operate light valves 24B, 24G,and MR, which control the exposure for making color separationnegatives, positives or printers in any suitable manner such as thatdescribed by Murray and Morse,

According to the present invention, a portion of each signal is passedthrough an Nth power amplifier shown respectively at 213, 21G and 21R toform high contrast positive color separation records of the original.These records are then added by any suitable adder 28 which may consistof a simple resistance across which the three signals are additivelyimpressed. Any method of adding the three signals linearly may beemployed. This combined high contrast signal is then passed into a Nthroot amplifier 29 and through a suitable linear amplifier 30 to operatea light valve 3|. Depending upon the arrangement of this light valve, itmay be used in the usual manner to control the scanning exposure for ablack printer negative or for a black printer positive.

With the arrangement shown the negatives or positives produced by thelight valves 243, MG, and MR would be those intended for a three colorprocess and would include that portion 01 the record which has also beenassigned to the black printer produced by the light valve 3|. As pointedout above, a tour-color process preferably is arranged so that theyellow, magenta and bluegreen printers are reduced in density by theamount assigned to the black printer. This may be done in the mannertaught by Murray and Morse in their application mentioned previously, byplacing in each channel a modifier controlled by the final black signal.For example, modifiers in the blue, green and red channels may beconnected to the leads 32, in the same manner as the modifier 25 isconnected to the amplifier 22G.

Having thus described in detail two embodiments of my invention and themanner whereby the invention fits into the development of black printersin general, I wish to point out that the invention is not limited tothese specific embodiments but is of the scope of the appended claims.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. For use in a four-color photo-mechanical process corresponding to athree-color process in which the blue-green,.magenta and yellow printershave respective printing transparencies at corresponding points withvalues Tba, Tm, and T11, a black printer whose printing transparency ateach corresponding point is when N is a number greater than 1.

2. For use in a four-color photo-mechanical process corresponding to athree-color process in which the blue-green, magenta, and yellowprinters have printing transparencies at corresponding points withvalues Tbg, Tm, and Ti! respectively, a black printer whose printingtransparency at each corresponding point is T...-+T..-+T.-

where N is a number greater than 1 and P is a number between 1 and 3.

3. For use in a four-color photo-mechanical process corresponding to athree-color process in which the blue-green, magenta, and yellowprinters have printing transparencies at corresponding points withvalues Thy, Tm and Ty, respectively, a black printer whose printingtransparency at each corresponding point is where N is a number greaterthan 1.5 and P is a number between 1 and 1.5.

4. A black printer negative for use in a photomechanical process inwhich the red, green, and blue separation negatives have densities D1",D9 and Db respectively, at corresponding points, said black printernegative having at each corresponding point a density equal to where Nis a number greater than 1.5.

5. In a photo-mechanical process for reproducing a multicoloredoriginal, the method of makinga black printer negative which comprisesforming red, green, and blue positive separation records of the originalwith a contrast N with respect to the original where N is a numbergreater than 1, adding the three records linearly to give a combinationrecord, forming from this combination record a photographic negativehaving a contrast of substantially 1/N with respect to the combinationrecord.

6. A method of making a black printer negative according to claim 5where N is a number greater than 1.5.

'1. In a photo-mechanical process for reproducing a multi-eoloredoriginal, the method of making a black printer negative which comprisesforming red, green and blue positive separation records of the originalwith a contrast N with respect to the original where N is a numbergreater than 1, adding linearly an equal fraction,

of each of the records to give a combination record, where P is a numberbetween 1 and 3., forming from this combination record a photographicnegative having a contrast substantially l/N with respect to thecombination record.

, 8. A method of making a black printer negative according to claim 5wherein the step of forming the positive separation records of theoriginal includes color correction of the type wherein at least onerecord is reduced in proportion to the intensity of at least one otherrecord.

9. A- method of making a black printer negative according to claim '7wherein the step of forming the positive separation records of theoriginal includes color correction of the type wherein at least onerecord is reduced in proportion to the intensity of at least one otherrecord.

of said high contrast records linearly to give a combination recordwhere P is a number between 1 and 3, forming from this combinationrecord a negative having a contrast substantially 1/N with respect tothe combination record and printing a printer from this negative.

11. In the process of making a black printer for use in a four-colorphoto-mechanical process for reproducing a multicolored original, thesteps which comprise forming high-contrast positive primary-colorseparation records of the original having a contrast N with respect tothe original where N is a number greater than 1, adding these recordslinearly to give a combination record and forming a negative from thiscombination 1'80? 0rd with a contrast substantially l/N with respect tothe combination record, whereby a negative suitable for making a blackprinter is produced.

7 12. In a four-color photo-mechanical process for reproducing amulticolored original,. the method of making a black printer negativewhich comprises exposing three photographic layers respectively to eachof the primary colors from the original to form color separationnegatives of the original, printing from. these negatives threephotographic'layers, respectively processing these latter layers to highcontrast to form color separation positives-having a contrast N I withrespect to the original where N is a number greater than 1, successivelyexposing a photographic layer in printing relation to each of the threepositives, and'processing this triply exposed layer to a contrastsubstantially .i/N with respect to the positives to form a negative.

13. In a four-color photo-mechanical process for reproducing amulticolored original the meth-! 0d of making a black printer whichcomprises making a negative according to claim 12 and printing apositive printer therefrom.

14. The method of making a black printer negative according to claim 12where N is a number greater than 1.5. p

15. The method of making a black printer negative accordingto claim 12in which at least one color correction mask is made from one or thecolor separation negatives of the original and placed in register withanother of said color separation negative during the step of printing aphotographic layer from this latter negative.

16. In an eiectro-optical system carrying three primary color signals'inseparate channels for color reproducing, the method of making a blackprinter including the step of forming a black printer signal which stepcomprises amplifying an equal portion of each of the three primary colorsignals by an Nth power amplifier where N is a number greater than 1,adding the amplifled signals linearly to form a combined signal andamplifying the combined signals by a l/N power amplifier to give a blackprinter signal, the step of operating a light valve by this signal andthe step of forming a black printer separation record in accordance withthe response of the light valve.

17. The method of making a black printer negative according to claim 16where N is a number greater than 1.5.

JOHN A. C. YULE.

